Counter-rotating pairs of eccentric weights are often used to generate vibratory forces for driving elongate members such as piles, caissons, and the like. As is well-known in the art, when eccentric weights are rotated in-phase in opposite directions at the same speed, the transverse force components of each pair of eccentric weights cancel each other, while the longitudinal force components of each pair of eccentric weights are summed together.
Problems may arise when the counter-rotating weights are accelerated and/or decelerated. In particular, the counter-rotating weights are typically used as part of a larger driving system including a vibro housing, a clamp assembly, a suppression assembly, and a support assembly. The counter-rotating weights are mounted within the vibro housing. The vibro housing is secured to the clamp assembly and the suppression assembly. The clamp assembly rigidly secures the vibro housing to the elongate member to be drive, while the suppression assembly inhibits transmission of the vibratory forces from the vibro housing to the support assembly.
The longitudinal vibratory forces have a frequency determined by the rotational speed of the eccentric members. As the counter-rotating weights are accelerated and decelerated, the frequency of longitudinal vibratory forces may move through a range of frequencies that may cause other components of the driving system to resonate. When the driving system or portions thereof resonates, the driving system and the elongate member connected thereto can become unstable and/or become damaged. Ideally, resonant vibration upon starting and stopping of the driving system should be avoided.
The Applicant is aware of a class of eccentric vibration systems that may be used in a manner that avoids resonant vibration during starting and stopping. In particular, variable moment eccentric vibration systems allow the operator to vary the moment of the vibration system during operation. Variable moment eccentric vibration systems can be operated such that resonant frequencies during starting and stopping are avoided.
Variable moment eccentric vibration systems are, however, expensive and complex and require significantly more maintenance than conventional eccentric vibration systems. The need thus exists for eccentric vibration systems that avoid resonant frequencies during starting and stopping but are inexpensive and simple to operate and maintain.